Miniaturized electric source having a radioactive heat source

ABSTRACT

A MINIATURIZED ELECTRIC SOURCE HAVING A RADIOACTIVE HEAT SOURCE AND THERMOELEMENTS ENCLOSED BY INSULATING LAYERS, SAID THERMOELEMENTS BEING CONSTITUTED BY THIN METALLIC LAYERS ON TAPE CARRIERS ON INSULATING MATERIAL, SUCH CARRIERS BEING WOUND OR FOLDED TO FORM A COMPACT UNIT COMPRISING A HIGH NUMBER OF THERMOELEMENTS.

June 19, 1973 ADLER ETAL 3,740,273

MINIAIUHIZI'J'U ELECTRIC SOURCE HAVING A RADIOACTIVE HEAT SOURCE Filed Jan. 19, 1970 FIG.1 G

I 3 I 1 c 1N VENTOR.

United States Patent Int. Cl. (321d 7/00 US. Cl. 136-202 7 Claims ABSTRACT OF THE DISCLOSURE A miniaturized electric source having a radioactive heat source and thermoelements enclosed by insulating layers, said thermoelements being constituted by thin metallic layers on tape carriers of insulating material, such carriers being wound or folded to form a compact unit comprising a high number of thermoelements.

This invention relates to a miniaturized electric source comprising a radioactive material in a hot source and thermoelements enclosed by insulating layers.

In a prior electric source of the above type one surface only of the hot source may be covered with thermoelements due to screening, this resulting in a low efiiciency.

It was further proposed to increase the length of the thermoelements in order to increase the temperature difference between the hot and cold connection of the thermoelements. However, due to the increase of the internal resistance of the thermoelements by its increase in length no higher voltage is obtained at the terminals under normal operation. Particularly, when using Bi Te -thermoelements with a coeificient of heat conduction of 9 a higher total heat energy is needed in order to obtain a higher temperature drop.

Further the number of thermoelements per unit area of prior electric sources is much too small for obtaining voltages which may directly be used for operating electronic circuits.

This invention aims in overcoming the drawbacks of prior electric sources by providing thermoelements formed of thin-film layers applied to an electrically and thermical- 1y isolating carrier and disposed on said hot source along a curve. The carrier with thermoelements formed thereon may preferably be wound in a spiral curve or folded in a meander line.

This invention will now be explained in more detail with reference to the accompanying drawing iHustrating, by way of example, two embodiments of the invention.

FIG. 1 is a sectional view of the first embodiment,

FIG. 2 is a sectional view of the second embodiment, and

FIGS. 3 and 4 are perspective views of a wound unit and folded unit respectively of thermoelements.

The electric source illustrated in FIG. 1 has a source 1 of radioactive material enclosed in a sealed metal casing 2. Casing 2 is surrounded by an electrically insulating layer 3 of good heat conductivity. All surfaces of the hot source constituted by parts 1 to 3 are covered by units 4 comprising as many thermoelements as possible with their hot connections in contact with layer 3 and their cold connections covered with another electrical insulation 5 contacting an outer casing 6 serving as a cooler.

Preferably the thermoelements are produced in miniaturized thin-film form by methods usual in microfilm technique. In this way it is possible to accommodate a very high number of elements per unit area, this resulting in a relatively high output voltage in spite of a relatively low temperature drop between the hot and cold ends of the thermoelements. The thermoelements may also be produced of metals by vapour deposits on an insulating carrier in tape form and such tapes may be wound or folded to form units of thermoelements to be applied to the surfaces of the hot source. The tape may be wound in spiral form or folded in meander shape. Such a unit may be connected to the insulating layer 3 or, if a rim portion of the insulating carrier is free of conducting portions of the thermoelements, this rim portion may completely replace the insulating layer 3 and the unit may directly be applied onto the metallic casing 2. In this case, the outer insulating layer 5 may also be omitted and heat transfer between the thermoelements and the outer casing 6 may be improved thereby. In this way an extremely small section of the thermoelements is feasible in the order of 10 X a. With an available surface of 6 cm? about 10 thermoelements may be accommodated. An output voltage in the order of 4 v. may be obtained, this being proper for operation of electronic circuits. With prior sources a voltage in the order of 100 mv. was available and this voltage had to be transformed by means of a DC-DC-converter into a higher operating voltage of say 9 v. The appreciable losses of such a converter may thus be avoided.

In order to obtain a high life time of the source in the order of 20 years as an example, plutonium 238 may be used as a radioactive material, and this source should be enclosed in a casing of tantal or platinum, this casing being covered by an electrically insulating layer with e l0. The thermoelements may be combined of Bi Se PbTe 0r SbBi. Metallic thermoelements have advantages over semiconductor-elements. Metals are more suitable for vapour depositing thin layers and are practically not subject to aging even under radiation, particularly gamma radiation. Therefore, metallic thermoelements are preferred over semiconductor elements.

In order to obtain perfect contact and heat transfer, the wound or folded tape carrying the thermoelements may be pasted to the hot source.

In the embodiment of FIG. 2 corresponding parts have been designated with the same reference numerals as in FIG. 1. While in the embodiment of FIG. 1 all surfaces of the hot source are covered with units 4, only two opposite surfaces of the hot source of FIG. 2 are covered with units 4. Preferably the hot source 1-3 is of cylindrical shape and the units 4 applied thereto are wound as shown in FIG. 3 or 4. The hot source and units 4 are accommodated in a cylinder 7 of insulating material. The outer casing is composed of end discs 6 and a metal cylinder 6' flanged onto discs 6 to form a strong casing. This casing may be enclosed into a sealed and evacuated glass bulb with the terminals of the source passing through this glass bulb. A space is formed between the insulating cylinder 7 and metal cylinder 6', this space being preferably evacuated for preventing heat convection between insulating cylinder 7 and cylinder 6. The inner surface of cylinder 6' may further be coated with a white paint or metal layer in order to reflect heat radiation. In this way heat losses may be reduced to a minimum and the major portion of heat is conducted through the thermoelements. At least parts 6 of the casings 6, 6 may be made of tantal.

The unit of thermoelements may have another cross section, for instance as shown in FIG. 4, Whereinthe tape carrier for the thermoelements is folded in meander form. In this case, the thermoelements should be covered by an insulating film in order to avoid direct contact where the thermoelements are facing each other.

Typical characteristics of a practical embodiment as shown in FIG. 2 are as follows:

Thermic power mw 200-150 Voltage v 4 Electric power [LW 30-120 Temperature drop in each unit C 20 Diameter of hot source and units mm 15 What We claim is:

1. A miniaturized electric source comprising a radioactive material in a hot source and thermoelements separated from each other by insulating layers, said thermoelements being formed of thin-film layers applied to a fiat continuous carrier strip of electrically insulating and thermically conducting material, a compact unit of thermoelements comprising a number of thicknesses of said strip packed in mutually parallel position, said unit being disposed on at least one plane surface of said hot source along a curve with one edge of said strip adjacent said surface of the hot source.

2. An electric source according to claim 1, wherein said thermoelements and carrier respectively form a compact unit, this unit being applied to a surface of said hot source.

3. An electric source according to claim 2, wherein a tape-shaped carrier is folded in a meander line within said unit.

4. An electric source according to claim 2, wherein a tape-shaped carrier is Wound in a spiral line within said unit.

5. An electric source according to claim 1, wherein thermoelements are disposed on all surfaces of said hot source.

6. An electric source according to claim 1, wherein thermoelements are disposed on a part of the surfaces of said hot source while the other surfaces are covered by isolating means.

7. An electric source according to claim 6, wherein thermoelements are disposed on opposite surfaces of said hot source.

References Cited UNITED STATES PATENTS 3,189,765 6/1965 Danko et al 310-3 X 3,018,311 1/1962 Bagno et al 136225 3,344,289 9/1967 Knight 3103 2,864,012 12/1958 Thomas et al. 3103 3,272,658 9/1966 Rush 136-202 X CARL D. QUARFORTI-VI, Primary Examiner J. M. POTENZA, Assistant Examiner- US. Cl. X.R. 3103 A 

